A fuel cell converts chemical energy contained in fuel directly into electric energy by causing a fuel gas such as hydrogen and an oxidizer gas such as air to react electrochemically. One type of fuel cell is a polymer electrolyte fuel cell (PEFC), which uses a polymer electrolyte as an electrolyte.
The electrochemical reaction which takes place in the fuel electrode and oxidizer electrode is as follows.Fuel electrode: 2H2→4H++4e−  (1)Oxidizer electrode: 4H++4e−+O2→2H2O  (2)
In order to produce a favorable reaction in both the fuel electrode and oxidizer electrode, reaction gas (fuel gas and oxidizer gas) must be supplied evenly to a reaction surface (i.e. the surface of the electrolyte membrane. To supply reaction gas to the reaction surface evenly, ease of gas flow in a gas passage formed in the cell and ease of gas diffusion inside a gas diffusion layer must be ensured.
The gas diffusion layer is disposed in contact with a separator, in which a passage allowing gas flow is formed. More specifically, the gas diffusion layer contacts a rib defining the passage in the separator. A fuel cell stack is typically fastened together in the lamination direction by bolts or bands, and hence the gas diffusion layer receives a load (or pressure) from the rib of the separator. The region of the gas diffusion layer which contacts the rib and is subjected to a load from the rib is known as a compressed region (or rib-contacting part). A non-compressed region (or gas diffusion portion) of the gas diffusion layer which faces the passage in the separator does not receive the stack-fastening load (i.e. pressure from the rib) that is received by the compressed region. Hence the non-compressed region which faces the passage swells toward the passage side. As a result, the cross-sectional area of the passage decreases, inhibiting the gas flow through the passage, and swelling of the gas diffusion layer toward the gas passage must therefore be suppressed.
Furthermore, to ensure that the reaction within the reaction surface is even, a sufficient gas diffusivity is required not only in the non-compressed region, but also in the compressed region. The gas diffusivity differs within the gas diffusion layer according to the state of compression. Since the state of compression differs between the non-compressed region and compressed region, a difference arises in the gas diffusivity of the two regions. It is therefore desirable that the gas diffusivity be as uniform as possible in the non-compressed region and the compressed region.
Tokkai 2002-343379, published by the Japan Patent Office in 2002, discloses a technique of regulating the gas diffusivity within the gas diffusion layer. By pressing the gas diffusion layer in the direction of thickness during manufacture, the gas diffusivity within the gas diffusion layer is regulated to a design value.